Airbags are commonly used in large vehicles, as to support a vehicle cab on a chassis and to support a trailer on a frame, the airbags absorbing road shocks. A vehicle manufacturer commonly sets a predetermined optimum height for the airbag. An airbag valve, commonly referred to as a leveling valve, maintains a height close to the predetermined height. It does this by flowing air from a pressured air source to the airbag when the height falls below a predetermined height such as 5 inches for a cab airbag and 15 inches for a tractor airbag, and flowing air from the airbag into the atmosphere when the height has increased. A height zone, such as from 14.9 inches to 15.1 inches may be established within which there is no air flow into or out of the airbag. The pressured air source is commonly set at a pressure of perhaps 130 psi, and is used to operate brakes, a horn, a pneumatic seat, and other miscellaneous equipment on the vehicle. The optimum pressure of the airbag may be about 40 psi to 70 psi, the pressure varying with the load.
One type of leveling valve in widespread use is mounted on a vehicle frame and includes a disc that can pivot back and fourth as a handle inner end attached to the disc moves up and down, the handle outer end moving up and down with a vehicle axle. The disc carries inserts that are pressed tightly against a sealing surface of a main valve member that has a slot. As the disc pivots clockwise and counterclockwise, passages in the inserts move over ends of the slots to complete air paths that flow air into or out of the airbag. This type of airbag valve is described in U.S. Pat. Nos. 5,934,320 and 6,202,992.
When the disc is in its neutral position, the airbag height should be at the predetermined height (e.g. 15.0 inches). It is usually desirable that the disc pivot only a small angle such as 1.5° before the airbag is connected to one insert passage that leads to the pressured air or is connected to the other insert passage that leads to a dump (the environment). In one example, the valve handle is 8 inches long, the insert passages lie one-half inch from the disc axis, and the air is supposed to flow when the airbag height changes by about one-eighth inch. The valve parts must be maintained within close tolerances to assure that the air valve slot will not be connected to either insert passage until the disc is pivoted 1.5° in either direction from the initial, or neutral position. Accumulated tolerances in each of several different parts contribute to variations in the required disc pivot angles to connect the airbag to the pressured air source or the dump. If the accumulated tolerances are great enough, there can be a worst case scenario wherein the slot is simultaneously connected to both the pressured air source and the dump, causing continual leakage of air from the pressure source into the environment. To prevent this, the valve parts are made with high precision, resulting in increased cost; also, a greater angle than the optimum of perhaps 1.5° is often resorted to. It is noted that the pivoting of the valve in either direction before it begins a connection to the high pressure source or to the environment, is called the “dead zone” wherein no air flows into or out of the airbag.